EBU have been working hard for the last years to provide a recommendation and guidelines for broadcasters, to assure that the listeners experience an even loudness level at all times. The work has resulted in the recently published EBU recommendation 128 (EBU R128). It includes both specification to calculate loudness, and recommendation for levels (-23 LUFS). LUFS is the new scale for representing "loudness levels".
http://labs.radionova.no/2011/01/07/ebu-r128-library/

The is an ANSI C implementation free for any type of use and modification, that we can wrap or port into Essentia:

https://github.com/jiixyj/libebur128

Seems quite some people on hydrogenaudio.org have used it already approved it works correctly. An example of using the library: https://github.com/jiixyj/loudness-scanner